There is proposed a tire-action-force detecting device for detecting grounding reaction force acting on a tire, for controlling an anti-lock brake system (ABS) or a traction control system. This detecting device detects a slip ratio of a tire to thereby enable calculation of a road-surface friction coefficient. Here, the calculation of the slip or the like is executed by using torque acting on a tire and a detection value such as a rotation number of the tire, as described in Patent Document 1.                Patent Document 1: JP-2003-14563 A (US20030000316A1)        
In the above detecting device, the torque acting on a tire is detected without any problem. In contrast, the reaction force (vertical force) acting on a tire needs to be calculated using a tire rotation speed obtained from a relative rotation angle of the tire from a wheel speed sensor or the like, which thereby gives a load to a calculating unit. To perform a calculation following the tire rotation speed, a high-speed processor is required, which increases costs. Further, the calculating unit requires a given area or more, so that it is difficult to decrease the body size.